FIG. 1 illustrates a wind turbine 1. The wind turbine comprises a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a hub 6. The hub 6 is connected to nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in FIG. 1 may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are used in large scale electricity generation or on a wind farm for example. In the latter case, the diameter of the rotor could be as large as 100 metres or more.
Ice formation on wind turbine blades is a well known problem, as wind turbines are frequently installed in cold and stormy environments. The accrual of ice or other matter, such as dirt, is a hazard and leads to reduced wind turbine performance. It is a hazard because ice or other matter on the turbine blades may fall from the blades at any time, and in large amounts. It reduces wind turbine performance because it affects the aerodynamic behaviour of the blades and because the turbine may need to be stopped to remove hazardous ice or dirt.
The detection of ice on wind turbine blades has been achieved in a number of ways. One method that has been proposed is to monitor the bending loads on wind turbine blades.
It is known to provide the blades of a wind turbine with strain gauges in order to monitor the bending moment on the blades. This can be used in order to monitor the loads applied to the blades. Optical strain sensors, such as Fibre Bragg Grating strain sensors, are known for monitoring strain in wind turbine blades. Optical strain sensors for measuring the strain in wind turbine blades, and in particular for measuring the flapwise bending strain, are typically positioned at the root of the turbine blade. Measurement of flapwise bending strain of a wind turbine blade requires a measurement technique capable of distinguishing between strain on a strain sensor as a result of bending forces and strain resulting from other forces such as centripetal force. In order to do this, strain sensors are arranged pairwise around the root of the turbine blade, with the sensors in each pair arranged diametrically opposite each other. The strain due to bending detected by the sensors in each pair should be is approximately equal but of opposite sign, as one sensors will be under tension and one under compression. Strain due to centripetal force should be the same for both sensors. Using two pairs of sensors allows a bending strain to be determined in two dimensions, i.e. edgewise and flapwise. From changes in these bending strains, the build up of ice can be detected.
Although this method of measuring bending strain gives good results in theory, in practice it is not as precise as some applications need. This is the result of several factors. First, the material used to form the turbine blades is not absolutely homogenous. Second, the thickness of the material forming the turbine blades is not absolutely uniform. Third, the temperature of the wind turbine blade may vary slightly from one spot to another. Fourth, the sensors may not be mounted absolutely accurately. Fifth, in practice, sensors often fail or give erroneous results during their service lifetime.
We have recognised that there is a need for a more sensitive way of detecting the build up of ice or other matter on wind turbine blades.